Molecular basis for optical clearing of collagenous tissues

Abstract

Abstract. Molecular interactions of optical clearing agents were investigated using a combination of molecular dynamics ͑MD͒ simulations and optical spectroscopy. For a series of sugar alcohols with low to high optical clearing potential, Raman spectroscopy and integrating sphere measurements were used to quantitatively characterize tissue water loss and reduction in light scattering following agent exposures. The rate of tissue water loss was found to correlate with agent optical clearing potential, but equivalent tissue optical clearing was measured in native and fixed tissue in vitro, given long-enough exposure times to the polyol series. MD simulations showed that the rate of tissue optical clearing correlated with the preferential formation of hydrogen bond bridges between agent and collagen. Hydrogen bond bridge formation disrupts the collagen hydration layer and facilitates replacement by a chemical agent to homogenize tissue refractive index. However, the reduction in tissue light scattering did not correlate with the agent index of refraction. Our results suggest that a necessary property of optical clearing agents is hyperosmolarity to tissue, but that the most effective agents with the highest rates of optical clearing are a subset with the highest collagen solubilities.

Optical clearing mechanisms characterization in muscle

“…In particular, hydrogen bond bridge formation disrupts the collagen hydration layer and facilitates water replacement by the OCA to perform RI matching. 36 In vivo studies were also performed in dorsal rat skin to evaluate OC, OC mechanisms and protein dissociation. 37 These studies have demonstrated that no collagen¯bers were dissociated or fractured at used OCA concentrations.…”

An automated approach for three-dimensional quantification of fibrillar structures in optically cleared soft biological tissues

“…collagen) and the ground matrix of soft tissue (e.g. interstitial fluid), by replacing water in the tissue with a fluid of a similar index of refraction as that of the proteins [9][10][11]. Measurements based on spectroscopy techniques, polarization or optical coherence microscopy as well as nonlinear imaging techniques such as two-photon fluorescence and second-harmonic generation (SHG) yield more valuable, otherwise virtually inaccessible information when performed on cleared (versus uncleared) soft biological tissue [12].…”

Quantitative analysis of dehydration in porcine skin for assessing mechanism of optical clearing

“…The second one might be from tissue shrinkage (less thickness) and corresponding spatial correlation of scatterers (more effective packing or ordering) caused by dehydration, which also gives clearing effect due to constructive interference in the forward direction of all scattered waves by spatially correlated collagen fibers, as is characteristic to an absolutely transparent eye cornea. 28 The last mechanism should correlate with more regular packing or ordering of scatterers, 11 which increase the anisotropy factor g. 31 For the molecular mechanism of dehydration caused by OCAs, Hirshburg et al 32,33 gave a reasonable explanation with the molecular dynamical simulation and demonstrated that the dehydration process is involved with the propensity of forming hydrogen bond bridges, which can be evaluated by collagen solubility of OCAs. 32 Besides the dehydration, the optical clearing agents entered into the dermis may induce collagen-fiber dissociation 33 or alteration of scatterer structure, 31 which also enhanced the optical clearing efficacy of skin.…”

“…28 The last mechanism should correlate with more regular packing or ordering of scatterers, 11 which increase the anisotropy factor g. 31 For the molecular mechanism of dehydration caused by OCAs, Hirshburg et al 32,33 gave a reasonable explanation with the molecular dynamical simulation and demonstrated that the dehydration process is involved with the propensity of forming hydrogen bond bridges, which can be evaluated by collagen solubility of OCAs. 32 Besides the dehydration, the optical clearing agents entered into the dermis may induce collagen-fiber dissociation 33 or alteration of scatterer structure, 31 which also enhanced the optical clearing efficacy of skin. All these or some other possible mechanisms do not take effect independently but interact on each other and contribute to the ultimate clearing efficacy together.…”

“…The reason for the worse optical clearing efficacy caused by D-sorbitol could be from its low-saturated solubility (70%). Hirshburg et al 32 performed the molecular dynamics simulations of collagen peptides and found that the rate of tissue optical clearing correlated with the preferential formation of hydrogen bond bridges between agent and collagen. They indicated that glycerol prefers to form type II hydrogen bond bridges, whereas sorbitol prefers type IV bridges, which span more extensively across the collagen surface and lead to greater disruption of the collagen hydration layer than type II.…”